Sweet basil (Ocimum basilicum L.) is one of the most popular and economically important culinary herbs in North America and Europe. This plant species is used in fresh, dried, essential oil and ornamental horticultural industries. The greater Ocimum genus is characterized by a great variability in its genetics, genomic structure, morphology, and essential oil composition (Marotti et al., J Agric Food Chem. 44:3926-9, 1996; Simon et al., Basil: A source of aroma compounds and a popular culinary and ornamental herb. Pages 499-505 in: Perspectives on New Crops and New Uses. J. Janick, ed. ASHS Press, Alexandria, Va., 1999; Vieira et al., J. Am. Soc. Hortic. Sci. 128: 94-99, 2003; Vieira et al., Flavour Fragr J. 21:214-221, 2006; Koroch et al., Israel J. Plant Science 58:183-189, 2010). Their unique capacity to manufacture a wide array of volatile constituents through the phenylpropanoid and terpenoid pathways has generated extensive interest and research in basil's aromatic volatile synthesis and plant secondary metabolism. Each Ocimum spp. produces essential oils with varying compositions and levels of volatile constituents distinct to each genotype (chemotype) and conferring unique flavor/aroma profiles (Charles and Simon, J. Essential Oil Research 4:231-23, 1992; Simon et al., Basil: A source of essential oils. Advances in New Crops, Timber Press, Portland, Oreg. pp. 484-489, 1990; Simon et al., Basil: A source of aroma compounds and a popular culinary and ornamental herb. Pages 499-505 in: Perspectives on New Crops and New Uses. J. Janick, ed. ASHS Press, Alexandria, Va. 1999; Vieira and Simon, J. Economic Botany: 54(2):207-216, 2000; Vieira et al., Biochemical Systematics and Ecology, 29/3:287-3, 2001). Although the wide diversity in chemotypes is exploited in various specialty industries such as in essential oils, the major United States and European markets demand a specific flavor dominated by monoterpene linalool and complemented by lower concentrations of phenylpropenes such as methyl chavicol and other terpenes including 1,8 cineole (Marotti et al., J Agric Food Chem. 44: 3926-9, 1996; Simon et al., Basil: A source of aroma compounds and a popular culinary and ornamental herb. Pages 499-505 in: Perspectives on New Crops and New Uses. J. Janick, ed. ASHS Press, Alexandria, Va., 1999). In addition to the aroma/flavor, the physical appearance of the plant (phenotype) varies according to its utility in the 1) fresh; 2) processed consumer market; or 3) ornamental/home horticultural and potted plant markets. Within the processed market basil is primarily consumed as a 2A) frozen or 2B) dried or freeze dried product. It is generally observed that market 1B prefers a concave leaf shape, while markets 1A, 2A and 2B prefer a less concave ‘large leaf’ type. The large leaf type basil varieties are associated with high yield (i.e., foliar volume) with a leaf-to-stem ratio suited for this whereas the concave leaf type varieties tailored to market 1B are often less vigorous and have lower leaf-to-stem ratios, but are more aesthetically pleasing to the consumer. Recent breeding efforts have seen a proliferation of new basil varieties bred to meet the demands of these markets, both of which continue to grow.
Current sweet basil production is being threatened by basil downy mildew (BDM), the causal pathogen of which is Peronospora belbahrii (Belbahri et al., Mycol. Res. 109:1276-1287, 2005; Thines et al., Mycol. Res. 113:532-540, 2009). BDM has caused significant economic damage to the annual sweet basil market in the US, Europe, and in other regions worldwide (Hansford, Rev. Appl. Mycol. 12: 421-422, 1933, Garibaldi et al., Plant Dis. 84:1154, 2004, Garibaldi et al., Plant Dis. 89:683, 2005, McLeod et al., Plant Dis. 90:1115, 2006, Khateri et al., J. Plant Pathol. 89:S70, 2007, Ronco et al., New Disease Reports 18:14, 2008, Safrankova and Hollova, Plant Dis. 98:1579, 2014) with annual losses to reduced acreage and failed crops estimated in the millions of dollars (Wyenandt et al., HortScience 45(9): 1416-1419, 2015). BDM was first identified in Uganda in 1933 (Hansford, Rev. Appl. Mycol. 12: 421-422, 1933), but was not observed in the USA until 2007 when its occurrence was reported in southern Florida. Since then the disease has been reported around the world, and was found to be spread naturally in air currents and via infested seed or transplants (e.g., seedlings) infested with the pathogen and disseminated throughout the EU and North America (Wyenandt et al., HortScience 45(9): 1416-1419, 2015). When environmental conditions are conducive to high disease pressure complete crop losses in both greenhouse and field operations have been reported (Roberts et al., Plant Dis. 93(2):199, 2009; Wick and Brazee, Plant Dis. 93(3):318, 2009; Cohen et al., 97(5):692, 2013; McGrath, Cornell University: Vegetable MD Online; expect and prepare for downy mildew in basil. vegetablemdonline.
ppath.cornell.edu/NewsArticles/BasilDowny.html, 2011; McGrath et al., Downy mildew wars: A monitoring program can help growers determine if the basil downy mildew pathogen is present in their area. American Vegetable Grower. February 2010 (4 pages); Wyenandt et al., Phytopathology 105:885-894, 2010). BDM development is rapid during periods of high humidity, mild temperatures, and extended periods of leaf wetness (Spencer D. M. (ed.) 1981. The Downy Mildews. Academic Press, New York; Garibaldi et al., Plant Dis. 89:68, 2005; Garibaldi et al., J. Plant Dis. Prot. 114:6-8, 2007). The asexual spores (sporangia) can be spread via wind current, water dispersal, infection/infestation of transplants and contaminated seed (Farahani-Kofoet et al., Mycol. Prog. 11:961-966, 2012). The unusual versatility by which this obligate parasite can be disseminated is a major reason for its global spread in recent years (Garibaldi et al., J. Plant Dis. Prot. 111:465-469, 2004; Belbahri et al., Mycol. Res. 109:1276-1287, 2005; Thines et al., Mycol. Res. 113:532-540, 2009).